This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
DoofyLuck, seemeemail1, bendydem0n, w.d.gastertejode, snyxnon, Trailblazer, ramsveenbalaba, cezardiri, osuruk-kokulubok, DrAhune, kaivanie, poi967, LMR_1, Captain PogChamp, holozler235, Ford Crown Victoria, Пожилая Скумбрия, V92, Ryuji, LazerSpy, andrewdbuilder